Ultrafine powders have significant potential for a wide range of applications including catalysts, magnetic recording media, optoelectronic materials, magnetic fluids and composite materials. Materials having fine-scale microstructures possess unique and technologically attractive properties, as shown by work on rapidly solidified metals, alloys and composite materials wherein grain sizes on the order of a few micrometers (microns) are obtainable. Recently, however, research has focused on reducing grain size from the micrometer to the nanometer range. A feature of such nanostructured materials is the high fraction of atoms (up to 50%) residing at grain or particle boundaries. The high fraction of the atoms residing in surface boundaries is important in creating a high density of potential sites for catalytic and electrochemical reactions. Nanostructured materials, which refer herein to materials having a grain diameter of about 1 to 100 nanometers (1 nm=10 angstroms) possess substantially different, and in many cases improved, chemical and physical properties compared to their micron-sized grain counterparts of the same chemical composition.
The synthesis of oxides and hydroxides in nanostructured form facilitates the manufacture of components and devices with modified and/or superior performance. An additional benefit of grain size refinement to the nanoscale range is the elimination of large voids at grain boundaries, which often contribute to deleterious properties. The recent demonstration that nanostructured ceramic materials (i.e., TiO.sub.2) can be superplastically deformed at relative low temperatures and pressures, as described in the examples in Nanostructured Materials, Vol. 1, 1992, has important implications in net shape forming brittle ceramic and intermetallic components. In industrial coating applications, for example thermal barrier coatings, reduction to the nanoscale is highly effective in increasing thermal impedance. In dense ceramic coatings, nanostructured materials have the potential to provide high hardness combined with good fracture toughness, and corrosion resistance.
High surface-area materials with nanoscale dimensions are of special interest in applications where active-site mediated chemical reactions play a decisive role. In catalytic applications, a high contact area for oxidation and reduction with the surrounding environment is important, and thus reduction of the catalytic material to the nanoscale is clearly a distinct advantage. Catalytic applications include pollution control, such as nuclear waste mitigation, water purification, mercury remediation, particulates remediation, and air filtration, as well as catalysis for synthetic purposes, such petroleum refinement, and the like. However, despite a strong interest in the development of nanostructured materials for catalytic applications, the present nanostructured materials suffer from the tendency of the particles to form agglomerates in which the interparticle pore space becomes comparable with the particle size, i.e. wherein the interparticle pore space itself has nanoscale dimensions.
Another area for the use nanostructured materials is rechargeable batteries and fuel cells, where the high surface area of the nanostructured materials promotes rapid interactions of the active material with the surrounding media. In high energy density rechargeable storage batteries, for example, the need to sustain high current pulses under charging and discharging conditions requires maximum contact between the electrode and electrolyte to achieve a high density of ion and electron carriers. Active nanostructured materials, with their high density of controlled surface defects, address this requirement, thus providing the means to optimize the high energy storage capacity of batteries.
Work on micron-scale nickel hydroxide-based materials indicates three forms of crystal structure, namely the hexagonal .alpha.- and .beta.-phases and the cubic .gamma.-phase. In nickel electrode applications, the .beta.-phase is usually used because of its stability during charge-discharge cycle process. However, .alpha.-nickel hydroxide, although unstable in the charge-discharge cycle, is capable of storing a higher amount of energy due to a higher valence charge. Current nickel electrodes are less than ideal because of the low volumetric energy density of the active materials. The theoretical x-ray density of nickel hydroxide is 4.15 g/cc, but the present electrode can only achieve a density of 1.8 g/cc. This is primarily due to the large micro-sized voids associated with the processed electrodes when using conventional nickel hydroxide.
The particle size in both naturally occurring and commercially synthesized manganese oxide is on the micron scale. Crystallographic studies reveal that at the molecular level manganese dioxide is constructed from octahedrons, each consisting of six oxygen atoms surrounding a manganese atom. The octahedrons are linked at their vertices and edges to form single or double chains. These chains share corners with other chains, leading to structures with tunnels or channels formed by rows of atomic empty sites. The size of these channels is related to the number of manganese-oxygen chains on each side. The presence of channels facilitates the transport of mobile ionic species, including Li+, H+, K+, Ba+2, Na+, or Pb+2. This feature is important because such cation exchange enhances both catalytic properties for oxidation reactions, and good ionic/electronic conduction for energy storage batteries.
Zirconia is another oxide of particular interest because of its chemical stability, high hardness, refractory nature (ability to withstand high temperatures), and ionic conductivity. Structurally stabilized zirconia is widely used in thermal barrier coatings for advanced engines, which are subject to extremely high temperatures. Other uses of zirconia include milling balls, refractors, oxygen sensors, and fuel cells batteries, as well as electronic ceramics.
Zirconia has a monoclinic structure at low temperatures, but exists in different forms at elevated temperatures. For example, undoped zirconia with the monoclinic structure transforms near 1170° C. to the tetragonal structure, and then near 2370° C. to the cubic structure. The presence of lower-valance cations such as Mg+2, Ca+2, Y+3, and rare earth cations stabilizes the high temperature phases to lower temperatures so that metastable tetragonal or cubic phases can exist down to ambient temperature. Also ZrO2 nanoparticles produced by sol-gel synthesis are heavily agglomerated.
As can be seen by the above discussion, despite recent developments in the synthesis of nanostructured materials, there still remains a need for materials and methods which are economical, and which produce materials which are suitable for a wide range of industrial applications. There remains a particular need for materials suitable for catalytic applications, i.e. materials having a high density of active sites.
A method for the manufacture of ultrafine powders with particle sizes less than 50 nm is described in International Application No. PCT/AU96/00539. This process involves a mechanically activated chemical reaction between a metal compound and a suitable reagent which occurs either during mechanical milling or during subsequent heat treatment of the milled powder. During mechanical activation a composite structure is formed which consists of nano-sized grains of the nano-phase substance within the matrix of the by-product phase. Removal of the by-product phase yields nano particles of the desired material.
The above described prior art techniques require the occurrence of a mechanically activated chemical reaction between the starting powders to form nano-sized particles. Ultrafine grinding processes such as attrition milling are known to be effective in producing powders with mean particle sizes down to about 500 nm. However, the achievement of smaller particle sizes generally requires long milling times and significant energy inputs and is therefore limited by economic considerations. Contamination of the product may also be a problem.
The advent of new methods to prepare semiconductor and metal nanocrystals, specifically the injection of molecular precursors into hot organic surfactants, has yielded markedly improved samples with good size control, narrow size distributions, and good crystallinity of individual and dispersable nanocrystals Using nonhydrolytic preparations of metal oxide nanocrystals at high temperature in organic surfactants, one observes markedly different properties with respect to defect structure and surface composition. Metal oxide nanocrystals with nonhydroxylated surfaces are believed to have significant advantages for applications in catalysis, ceramics, energy storage, magnetic data storage, sensors, ferrofluids, etc.
A method of increasing the refractive indexes of polymers is combining metal oxide particles with polymers to form ceramers. Ceramers may be defined as hardened or cured compositions having ceramic particles embedded or grafted into the polymer matrixes and typically having optical and physical characteristics intermediate between those of the metal oxide and the organic component. Transparency of ceramers is dependent upon, in part, the sizes and refractive indexes of the metal oxide particles contained therein. If the metal oxide particles used in ceramer preparations have large mean diameters, the transparency of the ceramers may be diminished. Light directed at the ceramer containing large metal oxide particles would be reflected back to the light source or deflected to the side, reducing the apparent transparency of the ceramer. Moreover, the transparency of the ceramer may be diminished upon the addition of very small metal oxide particles, if the particles agglomerate within the polymer. These particle agglomerates act as larger particles that may scatter or reflect light. In addition, during the preparation of ceramers, metal oxide particles may precipitate creating what appears to be separate layers(s) that have less than maximal transparency. For a transparent ceramer, the small metal oxide particles must be highly dispersed (non-agglomerated) in the polymer to avoid light scattering.
One method of ceramer preparation is to treat metal oxide particles with specific organic acids, or other surface active agents, before combining the metal oxide particles with polymer precursors. It is through that surface-active agents coat the surface of the metal oxide particles and create an organophilic interface with the polymer compositions. An adsorbed organic layer on the outside of the metal oxide particles sterically inhibits agglomeration resulting in greater metal oxide particle dispersion stability. Another method of increasing the dispersibility of metal oxides in polymer compositions is to electrostatically stabilize the metal oxide particles by forming aqueous colloids. The electrostatically charged layer surrounding the metal oxide particles provides repulsive forces between metal oxide particles inhibiting agglomeration. These methods of dispersing metal oxides suffer from various disadvantages. Consequently, transparent polymer/metal oxide compositions having high refractive indexes are desirable but difficult to obtain because the metal oxide particles, when combined with polymer precursors, may agglomerate and decrease the transparency of the resulting ceramers.
The refractive indexes of ceramers or ceramer compositions are, in part, dependent upon the refractive indexes of the metal oxide particles added to the organic matrix. The theoretical refractive index of a single ceramer can only be as high as the volume weighted average of the refractive indexes of the metal oxide particles and the polymer matrix. Consequently, it is desirable to make ceramers using metal oxide particles having high refractive indexes. Metal oxide particles in crystalline form typically have higher refractive indexes than metal oxide particles that are amorphous (that is, non-crystalline). Therefore, using crystalline metal oxides to make ceramers is desirable, though highly crystalline metal oxide particles that are dispersible are difficult to make because of their tendency to agglomerate in polymer compositions. Generally for a given particle size, the tendency to flocculate increases with increasing refractive index of the particles. For most high refractive index oxides, the form that is precipitated from solution is usually an amorphous or lower refractive index form. Treatments to induce crystallization (for example, heating at an elevated temperature) often tend to induce flocculation or agglomeration of the metal oxide particles. Heating also tends to induce sintering of aggregated of primary particles, especially very small particles that have very high surface areas and thus higher total surface energies. Because of its high dielectric constant, water is an excellent medium for crystallizing and stabilizing metal oxide particles. However, metal oxide particles that are formed in water are difficult to transfer into organic liquids without particle agglomeration and concomitant transfer of water.
Lithium based batteries have become commercially successful due to their relatively high energy density. Suitable positive electrode materials for lithium based batteries include materials that can intercalate lithium atoms into their lattice. The negative electrode can be lithium metal, lithium alloys or compounds that can reversibly intercalate lithium atoms into their lattice. Batteries formed from lithium metal or lithium alloy negative electrodes are referred to as lithium batteries while batteries formed with an anode (negative electrode) active material that can intercalate lithium ions are referred to as lithium ion batteries. In order to produce improved batteries, various materials have been examined for use as cathode (positive electrode) active materials for lithium based batteries. A variety of materials are useful in lithium based batteries. For example, vanadium oxides in certain oxidation states are effective materials for the production of positive electrodes for lithium based batteries. Also, metal vanadium oxide compositions have been identified as having high energy densities and high power densities, when used in positive electrodes for lithium based batteries. Silver vanadium oxide has a particularly high energy density and high power densities, when used in lithium based batteries. Silver vanadium oxide batteries have found particular use in the production of implantable cardiac defibrillators where the battery must be able to recharge a capacitor to deliver large pulses of energy in rapid succession, within ten seconds or less.
Nanoparticles have been reportedly made from metal carbonates such as ZnCO3. The production of such nanoparticles has been reported by methods such as chemical synthesis, gas-phase synthesis, condensed phase synthesis, high speed deposition by ionized cluster beams, consolidation, high speed milling, deposition and sol-gel methods. These methods suffer from numerous drawbacks, including agglomeration, broad particle size distribution, or low volume production.
In all the above-listed fields of industry there is a need for a cost-effective process for making nanoparticles of metallic compounds.